The UT-15, a chemical analogue of epoprostenol (prostacyclin, Flolan) will be effective for the treatment of vascular pulmonary hypertension in patients w/primary pulmonary hypertension, connective tissue disease, and congenital heart disease unresponsive to conventional therapy. SPECIFIC AIMS: The primary study is to determine whether UT-15 administered subcutaneously can improve exercise capacity, assessed by the Six Minute Walk Test. Secondary aims are to determine the effect of chronic administration of UT-15 on cardiopulmonary hemodynamics and on the clinical signs and symptoms of pulmonary vascular disease as well as to determine the pharmacokinetics of UT-15.